// 代码索引 4-1-4-1
#include <iostream>
#include <Eigen/Dense>
 
using Eigen::MatrixXd;
 
int main()
{
    MatrixXd A = MatrixXd::Random(100,100);
    MatrixXd b = MatrixXd::Random(100,50);
    MatrixXd x = A.fullPivLu().solve(b);
    double relative_error = (A*x - b).norm() / b.norm(); // norm() is L2 norm
    std::cout << "The relative error is:\n" << relative_error << std::endl;
}